1. Field of the Invention
The present invention relates to a method and apparatus for forming low dielectric constant (low k) polymeric films on a substrate, as an interlayer dielectric (ILD) material for fabrication of microelectronic device structures. The low k material may for example comprise parylene or a substituted derivative thereof.
2. Description of the Related Art
Copper currently is of great interest in metallization of very large-scale integration (VLSI) devices, due to its low resistivity, low contact resistance, and ability to enhance microelectronic device performance by reduction of RC time delays.
The concurrent use with copper metallization of a low dielectric constant material likewise provides reduction in the RC time constant, to further enhance device performance.
Among low dielectric constant materials, conventional SiO.sub.2 dielectric materials display values of the dielectric constant, k, near 3.2. For lower dielectric constant materials, potential candidate materials include polymers such as parylene as an interlayer dielectric (ILD) material. Some examples of the parylene family are given below: ##STR1##
Parylene displays a dielectric constant in the range of from about 2.2 to about 2.4. The use of parylene and/or related materials as interlayer dielectrics require that the dielectric material be easily deposited conformally over typical device topographies. Only then will increased device performance in the microelectronic device structure be realized, such as a VLSI device operating in the giga-hertz range in which the low k material is utilized as an ILD to electrically isolate signal lines, e.g., of copper, from each other.
A major manufacturing issue in the deposition of parylene-based dielectric materials, such as parylene-N, relates to the need for accurate and controlled delivery of the dimeric para-xylylene ([2.2]paracyclophane) as a starting material delivered to the thermal "cracking" chamber. In the cracking zone, reactive monomers and/or radical intermediates are formed for subsequent formation of the desired polymer. Co-reactants, such as cross-linking agents or co-monomers, may be added to form a specific polymer or to tailor the polymer properties, including the modulus, the dielectric constant, the thermal stability and the device performance properties.
Parylene-N films grow on a cooled substrate (at temperatures on the order of -20.degree. C.) in a vacuum environment by condensation of gaseous p-xylylene, a reactive monomer. The monomer, however, is not stable at room temperature conditions, but exists rather in the form of dimers. A vapor stream of the monomer can be readily generated by cracking the dimer vapor that is supplied from a stable crystalline solid dimer source. The dimer cracks at temperatures between 500 and 750.degree. C., with 600.degree. C. typically being employed for such dissociation.
Alternatively, the p-xylylene monomer can be created by cracking analogous compounds that contain a p-xylylene unit or derivative thereof.
Current dimer vaporization approaches employ the sublimation of the solid dimer as a feedstock to a cracking unit. Determined by the nature of a sublimation approach, the material transport rate of this process, however, is difficult to control. As a result, the stream of monomers exiting the cracking unit is not constant with time, which leads to an irreproducible deposition process. Liquid sources for parylene film formation may be vaporized in bubblers, but control of the vapor phase concentration of these low volatility compounds is difficult, and leads to poor reproducibility of the deposition process.
For microelectronic applications, control of the vapor concentration is critical, since it determines the growth rate, conformality of the resulting film, and reproducibility of the film formation process. In such applications, thickness must be tightly controlled for operability and adequate performance characteristics. Another problem associated with systems which couple pyrolysis with sublimation or bubbling is that the process equipment aggregately has a large footprint. For example, the process equipment may occupy an inordinately large volume of clean room space and thereby be undesirable for manufacturing environments.
It would therefore be a significant advance in the art to provide a method and apparatus for conveniently and economically providing low dielectric constant polymeric films such as parylene on a substrate with better reproducibility, and without the attendant problems of the prior art approaches for such film formation.
It therefore is the object of the present invention to provide such method and apparatus for the formation of low dielectric constant polymeric films on microelectronic device substrates, to enhance the device performance and reduce RC time delays.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.